1. Field of the Invention
This invention relates in general to earth boring drill bits and in particular to a large diameter shaft bit having an improved cutter arrangement.
2. Description of the Prior Art
Drill bits for large diameter shafts normally have a cutter support plate that is connected to a string of drill pipe for rotation. A number of cutter assemblies are secured to the cutter support plate to disintegrate the earth as the cutter support plate is rotated. The drilling may be downward, or upward by pulling the bit through a pilot hole, as shown in U.S. Pat. No. 3,805,901.
Normally the cutters are arranged to cut separate paths, although two or more cutters may be located in the same path for more effective earth disintegration. If two or more cutters are located in the same path, they must have different insert spacing patterns to avoid tracking. "Tracking" is a condition which results when a cutter tooth engages a previously made depression in a borehole bottom or face. As a result, a crest of rock may be generated on the face, which may lead to disadvantages such as erosion of the cutter shell or premature tooth disintegration. The different insert patterns require additional inventory and costs.
Another disadvantage of prior art drill bits is that the bit bodies may not be used both for tooth cutters and for discs. A disc cutter has a sharp circumferential ridge, rather than individual teeth. The best disc cutters utilize only a single ridge, since it has been found that double ridge disc cutters wear faster than single ridge cutters. However, the bearing and seal requirements normally require a fairly wide mounting bracket for tooth cutters. Mounting a single disc in a wide mounting bracket would place the disc too far from adjacent discs for the desired spacing.